1. Field of the Invention
This invention relates to an image display apparatus, which is preferably applicable for displaying a video image having an aspect ratio of 16:9 on a display screen having an aspect ratio of 4:3 or for displaying a video image having an aspect ratio of 4:3 on a display screen having an aspect ratio of 16:9, in particular, an image display apparatus in which a boundary line which occurs between a video image area and a non-video image area, due to the difference in luminous characteristics of phosphors of display elements is hardly recognizable.
2. Description of Related Art
A video display apparatus such as a television receiver using an existing picture tube (cathode-ray tube: CRT) or a video monitor has a display screen having an aspect ratio of 4:3 or 16:9 being a landscape monitor. As information of a video image to be displayed on the display screen, there is video information which shows circularity-1 when displayed by the aspect ratio of 4:3 (here-in after referred to as a 4:3-image) and video information which shows circularity-1 when displayed by aspect the ratio of 16:9 (here-in-after referred to as a 16:9-image).
Accordingly, circularity is not 1 when the video signal is displayed on the display screen having a different aspect ratio from that of the video signal. On this,account a size of the video image is changed or a non-image part is added to a spare space of the video signal in order to make the circularity to be 1 when the video signal is displayed on the display screen having the different aspect ratio.
For example, when the 16:9-image is displayed on the display screen having the aspect ratio of 4:3 in the video display apparatus, only 75% of a video image area Pv is scanned in the vertical direction of the display screen by a deflected electron beam, so that the size of the displayed image is compressed by 75% as shown in FIG. 8. Thereby, the circularity is maintained to be 1 and the 16:9-image is correctly displayed.
In this case, a top part and a bottom part of the display screen where the deflected electron beam does not scan becomes a non-video image area Nv where the phosphor does not emit light.
In addition, when the 4:3-image is displayed on the display screen having the aspect ratio of 16:9 in the video display apparatus, only 75% of a video image area Ph in the horizontal direction of the display screen is scanned by the deflected electron beam, so that the size of the displayed image is compressed in the horizontal direction by 75% as shown in FIG. 10. Thereby, the circularity becomes 1 and the 4:3-image is correctly displayed.
In this case, a left edge and a right edge of the display screen where the deflected electron beam does not scan becomes a non-video image area Nh where the phosphor does not emit light.
However, when a time period for displaying video images with the compressed image size, as shown in FIG. 8 or FIG. 10, becomes long hours, the luminous characteristics of the phosphors of the CRT become different between the video image area Pv and the non-video image area Nv or between the video image area Ph and the non-video image area Nh. Accordingly, when the display mode is changed, boundary lines between the video image area Pv and the non-video image area Nv or between the video image area Ph and the non-video image area Nh are recognizably left on the display screen of the CRT.
For example, in the case where the 16:9 image is displayed on the display screen having the aspect ratio of 4:3, the phosphor of the video image area Pv of the 16:9-image emits light in accordance with the video information, but the phosphor of the non-video image area Nv does not emit light at all.
Generally, the luminous characteristics of the phosphor is deteriorated if the duration of emitting time of the phosphor becomes long hours, so that the brightness for the video image area Pv of the 16:9-image becomes darker as compared with the brightness of the non-video image area Nv. As a result, the penumbra is recognized as a line.
After the image to be displayed is switched from the 16:9-image to the 4:3-image, a boundary line between the video image area and the non-video image area of the 16:9-image is left as the line and recognized.
Relation between a vertical position of the displayed image and the luminous characteristics of the phosphor is as shown in FIG. 9.
Similarly, in a case of FIG. 10, in which the 4:3-image is displayed on the display screen having the aspect ratio of 16:9, if such a condition is kept for a long time, a boundary line between the video image area Ph and the non-video image area Nh is easily recognized.
The present invention is made in consideration of the above problems and an object of the present invention is to make such a boundary line between the video image area and the non-video image area due to deterioration difference in luminous characteristics of phosphors hardly recognizable.
In order to solve the above-mentioned problem, this invention is to propose a video display apparatus, comprising: supply means for supplying display signals to a display device; mode control means coupled to said supply means for selecting display modes in said display device between a first display mode where an image signal having an aspect ratio of 4:3 is displayed on a display screen of said display device having an aspect ratio of 4:3 and a second display mode where an image signal having an aspect ratio of 16:9 is displayed on said display screen of the display device by compressing the image signal having an aspect ratio of 4:3 in a vertical direction; gray scale signal inserting means coupled to said supply means for inserting a gray scale signal into the image signal in the second display mode; and matching means for making a brightness level of the gray scale signal inserted into said image signal correspond to an average brightness level of said image signal.
According to the first aspect of the present invention, by inserting a gray scale signal into video signals on the head portion and the end portion of the vertical scanning period, it becomes hard to recognize the boundary line between the video image area and the non-video image area due to the difference in luminous characteristics of the phosphors in a case of the CRT, for example. It becomes possible to make the inserted gray scale signal not detectable in a case of an average brightness level of the video signals being zero by making the gray scale signal value correspond to the average brightness level of the video signals.
In another aspect of the present invention, there is provided a video display apparatus, which comprises supply means for supplying display signals to a display device; mode control means coupled to said supply means for selecting display modes in said display device between a first display mode where an image signal having an aspect ratio of 4:3 is displayed on a display screen of said display device having an aspect ratio of 4:3 and a second display mode where an image signal having an aspect ratio of 16:9 is displayed on said display screen of the display device by compressing the image signal having an aspect ratio of 4:3 in a horizontal direction; gray scale signal inserting means coupled to said supply means for inserting a gray scale signal into the image signal in the second display mode; and matching means for making a brightness level of the gray scale signal inserted into said image signal correspond to an average brightness level of said image signal.
According to the second aspect of the present invention, by inserting a gray scale signal into video signals on the head portion and/or the end portion of the horizontal scanning period, it becomes hard to recognize the boundary line between the video image area and the non-video image area due to the difference in luminous characteristics of the phosphors in a case of the CRT, for example. It becomes possible to make the inserted gray scale signal not detectable in a case of an average brightness level of the video signals being zero by making the gray scale signal value correspond to the average brightness level of the video signals.
Further in the another aspect of the present invention, it is presented a method for displaying image signal, comprises the steps of: supplying display signals to a display device; selecting display modes in the display device between a first display mode where an image signal having an aspect ratio of 4:3 is displayed on a display screen of the display device having an aspect ratio of 4:3 and a second display mode where an image signal having an aspect ratio of 16:9 is displayed on the display screen of the display device by compressing the image signal having an aspect ratio of 4:3 in the vertical direction; and inserting a gray scale signal on top and bottom of the image signal in the second display mode, wherein a brightness level of the gray scale signal is set to an average brightness value of the image signal.
Still another aspect of the invention, it is presented a method for displaying image signal, comprising the steps of: supply means for supplying display signals to a display device; selecting display modes in the display device between a first mode where an image signal having an aspect ratio of 16:9 is displayed on a display screen of the display device having an aspect ratio of 16:9 and a second mode where an image signal having the aspect ratio of 4:3 is displayed on the display screen of the display device; and inserting a gray scale signal on left and right of the image signal in the second display mode, wherein a brightness level of the gray scale signal is set to an average brightness value of the image signal.